1. Field of the Invention
This invention relates to an electric power circuit for telephone set use which utilizes a power drawn from a telephone circuit line, and more particularly to an electric power circuit capable of providing high voltage and large magnitudes of current.
2. Description of the Related Art
In accordance with the dramatic development of the semiconductor techniques in recent years there has been widely adopted a telephone set having a function such as a re dialing function that repeats a dialing of the telephone number once dialed or a dialing function by using an abbreviated telephone number which dials telephone numbers preliminary stored by entering corresponding abbreviated numbers, and the like. In such a telephone, a dialer IC is incorporated in order to perform a dialing; is, the dialer IC offers the dialing function.
A general structure of a conventional telephone set will be explained with reference to FIG. 7 of the accompanying drawings.
In FIG. 7, left side of a chain-dot line shows a telephone set 100 connected to a pair of telephone circuit lines L1 and L2 with connectors 10a and 10b. Since the connectors 10a and 10b are also connected to a diode bridge 12, a positive voltage will be delivered constantly from a positive output terminal 12a even though a path of positive voltage is interchanged between the two lines.
A dial pulse switch 14 is connected to the positive output terminal 12a of the diode bridge 12. When dialing a number, the dial pulse switch 14 generates dial pulses transmitted from the telephone circuit lines L1 and L2 by periodically interrupting direct-currents.
A voltage VL output from the dial pulse switch 14 is then fed to a speech network IC 16. The speech network IC 16 is connected to a handset 18 and produces an output of voice signals, which are entered from the handset 18, to the telephone circuit lines L1 and L2, and vice versa.
The voltage VL output from the dial pulse switch 14 is also supplied to a dialer IC 30 as a power. However, the voice signals, which are entered both from the telephone circuit lines L1 and L2 and from the handset 18 through the speech network 16, are superimposed on the voltage VL in the form of alternating current signals. Therefore, it is not preferable to supply the voltage VL to the dialer IC 30 as they are. To solve this problem, there is provided a circuit for supplying a direct current voltage VDD to the dialer IC 30 constantly at a predetermined value.
Specifically, one end of a supply line 20 is connected to the positive output terminal 12a of the diode bridge 12, while the other end thereof is grounded by way of a resistor R0 and a capacitor C0. The capacitor C0 constantly supplies a predetermined value of voltage to a node where the capacitor C0 and the resistor R0 are connected each other being independent of fluctuations of the voltage VL. As a consequence, it becomes possible to maintain the voltage at the node nearly as much as the voltage VL.
However, since the capacitor C0 has an impedance of nearly zero to alternating current components (hereinafter referred to as "a.c. components") on the supply voltage VL, a.c. components flow to the ground. To prevent this, as one of fundamental requirements for the telephone set 100, it is necessary to secure the presence of the impedance of 600 ohms or more at inside of the telephone 100, which is connected to the telephone circuit lines L1 and L2. For securing the impedance of 600 ohms or more and to prevent attenuation of a.c. components on the supply voltage VL, a resistor R0 is provided in the circuit. Although the resistance value required for the resistor R0 is 600 ohms or more, usually used a resistor having the value ranging 1k to 2k ohms.
Meanwhile, the node voltage between the resistor R0 and the capacitor C0 is fed to the dialer IC 30 as its supply voltage VDD by way of a schottky diode SD1 which is disposed for preventing a reverse current from a circuit on the following stage. A capacitor C1 for producing a constant voltage and a zener diode ZD are connected on the way between the line 40, through which the voltage VDD passes, and the ground. More specifically, the capacitor C1 is used for smoothing the voltage VDD, and the zener diode ZD is for maintaining the voltage VDD at a predetermined value or less.
The telephone 100 is further provided with a reserve battery ES for supplying a power to the dialer IC 30 in case no power is supplied from the lines L1 and L2 being disconnected to the telephone set 100. A direct current voltage at a desired level is delivered from the reserve battery ES towards the voltage supply line 40 through a resistor RS and a diode DS. The schottky diode SD1 is therefore disposed to prevent a power of the battery ES from flowing into the power line 20.
The dialer IC 30 is connected to keypads (not shown) and supplies a dialing signal entered from the keypads to both the speech network IC 16 and the dial pulse switch 14 so as to transmit DTMF signals or dial pulse signals to the lines L1 and L2.
In the aforementioned arrangement, the voltage VL taken out from the lines L1 and L2 is supplied to the dialer IC 30 in the form of direct current voltage VDD. Meanwhile, a.c. components superimposed on the supply voltage VL attenuates within a level determined by the resistor RO.
As has been mentioned above, according to the existing telephone set 100, the resistor R0 serves to obtain the direct current voltage VDD with the impedance maintained at a predetermined level or more.
According to the arrangement of the existing telephone set, however, the obtained voltage VDD is reduced as low a little as causing the voltage drop due to the resistor R0 when compared to the supply voltage VL. In result, the voltage VDD fed to the dialer IC 30 becomes too low to drive the IC, severely restricting the design and operation of the dialer IC 30.
Namely, there exists a standard that a voltage between the telephone circuit lines L1 and L2 should not exceed 6V at 20 mA though the voltage is subject to variations that are caused by changing lengths of telephone line. Accordingly, the voltage VL output by way of the dial pulse switch 14 is generally set to about 4V at a line current of 20 mA. In case a supply current to the dialer IC 30 is 1 mA and the resistor R0 is 1 k ohms, the obtained voltage will be about 3V or so.
In order to operate transistors embedded in IC, a voltage supplied thereto should preferably be increased to as high enough as to be operative. In case the supply voltage VDD is as low as about 3V, design flexibility of the circuit is consequently restricted. As another problem, according to the existing circuit, the more the current IDD fed to the dialer IC 30 increases, the more current drop is caused by the resistor R0, resulting in a small available current.